Fresh water, because we have four small tanks. We have one 5 gallon and 3 10 gallons. We have two male betas one in the 5 gallon and one in a 10 gallon. Then the other two tanks are community tanks, we do need to up grade to a 30 gallon and combine the two, but have not got around to it. Our tanks would be considered overstocked, but we do not have issues and keep everything going nicely.
One 10gal Community tank:
2 Sunset Platys
1 Corydora Albino
1 gold dust Molly
1 adult Black Molly and 10 babies
We ended up with babies after bringing home the fish, she was there less than a week and had babies, we had about 15, so only lost a few. We need to upgrade the tank to make room for them.